rbmichae:
Don't be surprised if the data is significantly slower than you're used to, however. We used T-mobile in Europe in 2015, and had very high speed data... but that's because T-mobile was running a "high speed in Europe" promo that is now over. On our recent trip (2 weeks ago), data speeds were absolutely capped at 128 kbps. Which is VERY slow. Don't expect social media apps to work well, or at all. Maps barely load, if at all (use Google Maps to pre-download areas, it helps a lot). And the poor speeds are compounded when you have marginal service. We still sought out Wi-Fi whenever possible.
Do you recall what types of networks your phone was connecting to? 4GLTE? 3G? 2G? That makes a big difference in speed.
With T-Mobile, your phone will always connect to the fastest network it can find (with a roaming partner, anyway). But T-Mobile still throttles your speed to slow it down. When I used T-Mobile last May-June in the Baltics, St. Petersburg, and Amsterdam, I connected mostly to 4G networks. I rarely noticed a slowdown...EXCEPT when I used a lot of data at once, say to stream a short video - then it would slow to a crawl for a few minutes. The rest of the time it seem zippy and fast. (After I came back, T-Mobile had their "summer of fast data promo" where they lifted the data throttling for the summer - but that didn't happen until after I got home.)
If your phone only connects to a 2G "edge" network, data will be impossibly slow whether they slow it down or not. Some phones may not have the right frequencies to connect faster than 2G in Europe, even if they are 4G phones in the US. (And some European countries use different frequencies than others.) I had a phone in Europe in 2015 that didn't have 3G or 4G European frequencies, so I could connect only at 2G, and it was challenging to use most of the time. That's wht makes a big difference in speed.
With T-Mobile, your phone will always connect to the fastest network it can find (with a roaming partner, anyway). But T-Mobile still throttles your speed to slow it down. When I used T-Mobile last May-June in the Baltics, St. Petersburg, and Amsterdam, I connected mostly to 4G networks. I rarely noticed a slowdown...EXCEPT when I used a lot of data at once, say to stream a short video - then it would slow to a crawl foy I upgraded my phone for 2016, and it worked so much better with T-Mobile (but this year, I dumped them anyway and bought the Vodafone SIM).